SSH and SCP support for Commons-Net

Apache Commons-Net has client-side implementations for many protocols, but the Secure SHell protocol is not supported. SSHv2 support will be added through this Summer of Code project. The challenge is a clean and extensible implementation that exposes a suitable API, and integrates well with [net]. This work will in turn be utilized to write SCP client classes, and set the ground for future support for SFTP.